The present invention refers to a process for the production of low carbon steel strips, having a good combination of strength and cold formability, as cast.
Different methods for producing carbon steel strips through twin roll continuous casting devices are already known. These methods aim at the production of carbon steel strips having good properties of strength and ductility.
In particular, in EP 0707908 A1 a twin roll continuous casting apparatus is shown and wherein a carbon steel strip is cast, for then undergoing in a hot rolling line with a 5-50% reduction of its thickness and being successively cooled. The flat thin product so obtained has good properties of strength and ductility thanks to the reduction in the grain dimension obtained with the hot rolling.
From WO 95/13155 an in line thermical treatment for cast carbon steel strips aiming at the control of a strip microstructure as cast is shown. In particular, the cast strip is cooled below the temperature wherein the transformation of austenite into ferrite occurs and the strip is successively heated until the material is riaustenitized (in line normalizing). In this way, for the effect of a double transformation phase into the solid phase, the austenitic grains become thinner, and by controlling the conditions of the final cooling and of the coiling of the strip it is possible to develop quite thin structures having good strength and ductility.
However, the above mentioned processes require further installations and higher energy consumption (e.g. rolling lines, furnace for intermediate heating etc.) and usually require larger space, and therefore less unity of the whole installation from the casting machine to the coiling reel. Furthermore, the object of the processes aim at the thickness of the final structure of the strip, trying to make it as similar as possible to that of a hot rolled strip from a conventional cycle, and they do not teach how to obtain a product with the desired mechanical and technological properties, by exploiting the peculiarities of the phase transformation features for the as cast steels with big austenitic grain (usually 150-400 microns).
Therefore, an object the present invention is to provide a process for the production of low carbon steel strips having, as cast, a good combination of strength and ductility and a good weldability, without undergoing rolling and/or thermal cycling stages.
Another object of the present invention is to provide a carbon steel strip which has, as cast, improved mechanical properties, in particular a relatively low yield/fracture stress ratio and a continuous pattern of the tension-strain curve, in order to make the material particularly suitable for cold molding applications such as bending and drawing.
Therefore, an object of the present invention is a process for the production of low carbon steel strips having a good combination of strength and formability, as cast, and a good weldability after pickling by the usual processes, comprising the following steps:
casting, in a twin rolls continuous casting machine comprising pinch rolls, a strip with a thickness comprised between 1 and 8 mm, having the following composition as weight percentage of the total weight:
C 0.02-0.10; Mn 0.1-0.6; Si 0.02-0.35; Al 0.01-0.05; S less than 0.015; P less than 0.02; Cr 0.05-0.35; Ni 0.05-0.3; N 0.003-0.012; and optionally, Ti less than 0.03; V less than 0.10; Nb less than 0.035, the remaining part being substantially Fe;
cooling the strip in the area comprised between the casting-rolls and the pinch rolls;
hot deforming the strip cast through said pinch rolls at a temperature comprised between 1000 and 1300xc2x0 C. until reaching a thickness reduction less than 15%, in order to encourage the closing of the shrinkage porosities;
cooling the strip at a speed comprised between 5 and 80xc2x0 C./s down to a temperature comprised between 500 and 850xc2x0 C.; and
coiling in to a reel the so obtainable strip.
In the process of the present invention, the phase transformation features of coarse grain austenite, which formed during the continuous casting process without performing hot rolling and/or in line normalizing, are exploited to produce by a controlled cooling and coiling, predetermined volume divisions of the microstructure constituents in the material as cast in low carbon steels. These final microstructures, constituted by equiaxed ferrite, acicular ferrite and/or bainite, provide a typical stress-strain diagram, of the material, with a continuous pattern, having an improved deformability as to make the strip particularly suitable for the applications in cold molding.